(function($) {
  $.fn.fullBg = function(){
    var bgImg = $(this);

    function resizeImg() {
      var imgwidth = bgImg.width();
      var imgheight = bgImg.height();

      var winwidth = $(window).width();
      var winheight = $(window).height();

      var widthratio = winwidth / imgwidth;
      var heightratio = winheight / imgheight;

      var widthdiff = heightratio * imgwidth;
      var heightdiff = widthratio * imgheight;

      if(heightdiff>winheight) {
        bgImg.css({
          width: winwidth+'px',
          height: heightdiff+'px'
        });
      } else {
        bgImg.css({
          width: widthdiff+'px',
          height: winheight+'px'
        });
      }
    }
    resizeImg();
    $(window).resize(function() {
      resizeImg();
    });
  };
})(jQuery)